High-Pressure and High-Temperature Synthesis and Pressure-Induced Simultaneous Optimization of the Electrical and Thermal Transport Properties of Nonstoichiometric TiO_1.80

Abstract

We developed suitable high-pressure and high-temperature (HPHT) conditions for improvement of the thermoelectric properties of nonstoichiometric TiO. X-ray diffraction, scanning transmission microscopy, transmission electron microscopy, and ultraviolet spectral measurements demonstrate that the crystal structures and microstructures are strongly modulated by our HPHT. The electrical properties and thermal conductivity are improved simultaneously by raising the reactive sintering pressure. The band gap was narrowed, contributing to the increase of the electrical properties with the pressure. In addition, relatively low thermal conductivities were obtained here as a result of a full spectrum of phonon scattering, benefiting from our deliberately engineered microstructures via HPHT. As a consequence, a high dimensionless figure of merit (zT) of 0.36 was obtained at 700 °C in the sample fabricated at 5 GPa. As far as we know, this is higher than all of the results of nonstoichiometric titanium oxide by other means and an enhancement of 57% of the best ever result. HPHT offers us a promising alternative for optimization of the thermoelectric properties, and it is worthwhile to popularize it.

Strategies and challenges of high-pressure methods applied to thermoelectric materials

“…150,151 It was found that non-stoichiometric samples of rutile (TiO 1.8 ) prepared under HP-HT conditions attain a relatively high ZT ∼ 0.35 at 700°C for a synthesis pressure of 5 GPa. 148,149 For BiCuSeO adopting a ZrCuSiAs-type structure (P4/nmm space group), a pressure-driven enhancement of the power factor at 700 K was predicted. 150 Later, it was experimentally shown that HP-HT synthesis at 3 GPa significantly improves the figure of merit of BiCuSeO, which reaches ZT ∼ 0.4 at 800 K. 151 For the halide thermoelectric BiTeI, a non-monotonic pressure dependence of the bandgap was predicted, 152 and it was proposed that its power factor may be enhanced many fold at room temperature under appropriate combinations of initial charge carrier concentration and applied pressure.…”

“…High-pressure effects on the thermoelectric parameters of many other families of potential thermoelectric materials have also been considered in the literature. For example, among oxide materials that may be of interest for their thermoelectric properties, some attention has been paid to rutile, TiO 2−x , 148,149 and BiCuSeO. 150,151 It was found that non-stoichiometric samples of rutile (TiO 1.8 ) prepared under HP-HT conditions attain a relatively high ZT ∼ 0.35 at 700°C for a synthesis pressure of 5 GPa.…”

Effects of Pressure on the Microstructure and Simultaneous Optimization of the Electrical and Thermal Transport Properties of Yb0.5Ba7.5Ga16Ge30

“…As expected, the zT value of n-type polycrystalline clathrates Ba 8 Ga 16 Ge 30 reached 1.14 at 773 K by the HPHT method . In comparison with the conventional approaches, the HPHT method offers a new degree of freedom beyond the temperature and chemical composition for optimization of the thermoelectric properties and broadens the breadth of opportunities of obtaining good thermoelectric performance . It is identified as an excellent substitutive method, which can effectively improve the TE performance of a large proportion of TE materials, such as oxide, PbTe, skutterudites, − and Bi 2 Te 3 .…”

Effect of Pb Filling and Synthesis Pressure Regulation on the Thermoelectric Properties of CoSb3

“…17 This is mainly due to that the nucleation rate of crystals will increase with the increase of synthesis pressures significantly. 18,19 Under the same synthesis time and the same element content, the number of grains synthesized under 3.5 GPa is larger than that of Pb 0.2 Co 4 Sb 11.5 Te 0.5 synthesized under 1.5 GPa, while the size of grains is smaller than that of Pb 0.2 Co 4 Sb 11.5 Te 0.5 prepared under 1.5 GPa. In order to investigate the chemical homogeneity of Pb-filled sample, we mapped the elemental distributions of Pb 0.2 Co 4 Sb 11.5 Te 0.5 prepared at 3.5 GPa by EDS in Figure 2c− 2g.…”

“…Figure a,b show the SEM micrographs of Pb 0.2 Co 4 Sb 11.5 Te 0.5 samples prepared at 1.5 and 3.5 GPa. The grain size of Pb 0.2 Co 4 Sb 11.5 Te 0.5 prepared at 1.5 GPa is smaller than that of Pb 0.2 Co 4 Sb 11.5 Te 0.5 prepared at 3.5 GPa, which is corresponding to the results of Yao et al This is mainly due to that the nucleation rate of crystals will increase with the increase of synthesis pressures significantly. , Under the same synthesis time and the same element content, the number of grains synthesized under 3.5 GPa is larger than that of Pb 0.2 Co 4 Sb 11.5 Te 0.5 synthesized under 1.5 GPa, while the size of grains is smaller than that of Pb 0.2 Co 4 Sb 11.5 Te 0.5 prepared under 1.5 GPa. In order to investigate the chemical homogeneity of Pb-filled sample, we mapped the elemental distributions of Pb 0.2 Co 4 Sb 11.5 Te 0.5 prepared at 3.5 GPa by EDS in Figure c–f.…”